This invention relates to millimeter wave radiometers, and more particularly to a lightweight, low power, large aperture millimeter wave radiometer with active image width control and vibration compensation for use in an airborne millimeter wave targeting and imaging sensor system.
It has long been recognized that millimeter wave sensors have the potential to play an important role in an aircraft-based all weather, day/night surveillance and reconnaissance system. Such a system detects passive microwave radiation emitted from objects on the ground and uses them to form an image of the area. Existing systems employ electro-optic (EO), infrared (IR) and synthetic aperture radar (SAR) technology, with SAR generally providing the best all weather capability. Conventional SAR systems have several drawbacks, however, principally shadowing and the limitation of being unable to look at or near nadir.
Frequently, a surveillance and reconnaissance system will include active microwave radiation sensing. An active microwave sensor receives the backscattering which is reflected from a transmitted microwave beam which is incident on the ground surface. Depending on the size of the transmitted microwave beam, an active microwave sensor can be used to provide an enhanced image of an area or for target designation. For target designation, a microwave signal is formed into a small beam and used to illuminate a specific target which can be followed by an advanced ordnance onto the target. Conventional SAR cannot be used as an all weather target designator due to the limitations of forming a real beam on the ground with a small antenna. A downward-looking millimeter wave (MMW) imaging system could fill in the operational holes in current sensor suites.
However, MMW imaging systems have suffered from a number of limitations, primarily, a combination of size, weight, power and frame rate limitations. Also, because of the need to have a long wavelength and thus large area apertures, MMW imaging systems have limited ground spatial resolution. Large, thinned arrays mounted on the airframe of a reconnaissance aircraft address some of these issues, but suffer from poor signal to noise ratio and the effects of wing motion.
To successfully function as a good all-weather imaging and targeting sensor system, a millimeter wave imaging system must simultaneously satisfy three competing requirements. First, the system must have adequate scene contrast. Adequate scene contrast for a passive system means small temperature sensitivity .DELTA.T, and for an illuminated (or active) system, a high signal to noise ratio. Second, the system must have adequate spatial resolution with acceptable point spread function (PSF) and side lobe levels. Grating lobes are a major issue in achieving a given resolution without ghosting. Finally, the system must have adequate field of regard (FOR).
Simultaneously meeting these requirements in a millimeter wave sparse aperture system for surveillance applications has proved difficult in the past. Obtaining a wide, instantaneous field of view (FOV) at millimeter wavelengths requires the use of small antennas (approximately millimeters in diameter). For apertures of more than a few meters, this means thousands of receiver antenna elements are necessary in order to maintain acceptable grating lobe levels and sensitivity. Because of the cost, weight, power and computational load (computations scale as the square of the number of receiver elements), such a millimeter wave sparse aperture system has been impractical to build and operate.
Thus, there is a need for a millimeter wave imaging system having small temperature sensitivity and high signal to noise ratio. There is a need for a millimeter wave imaging system which allows a relatively small number of large individual antenna elements in the array while retaining a large total field of regard. There is a need for a millimeter wave imaging system which compensates for motion of the airborne vehicle. There is a need for a millimeter wave imaging system which can accommodate active imaging. There is a need for a millimeter wave imaging system which includes a target designator subsystem having adequate spatial resolution with acceptable point spread function and side lobe levels. There is a need for a millimeter wave imaging and target designation system having an adequate field of regard.